Handle cloud corruption only for active store, improve error logging, remove NSFileCoordination, misc fixes.

[FIXED]     Only handle corrupt cloud content when it's from the active store.  Otherwise it's somebody else loading a store and shouldn't affect our store.
[ADDED]     NSStringFromUSMCause to describe a UbiquityStoreErrorCause.
[UPDATED]   Better logging of error conditions that have no NSError object.
[UPDATED]   Error log now describes cause.
[REMOVED]   NSFileCoordinators - are they even necessary?  In iOS 7 they started failing and they're probably not a good idea on SQLite files to avoid conflicts with the iCloud framework.
[ADDED]     NSPersistentStoreRemoveUbiquitousMetadataOption when migrating a cloud store.
[FIXED]     Only delete the migrationWorkStoreURL if it's a copy.